The present invention pertains generally to cryogenics and more particularly to inertially driven fusion (laser, electron beam, ion beam, etc.) target fabrication.
A number of techniques, offering varying degrees of success, have been used to produce cryogenic fuel cores within glass microballoon spheres. The difficulties associated with forming a uniform liquid or solid layer of material such as DT, arises from the smallness of the fuel core, the effects of gravity, and the strong influence played by surface rather than by bulk properties of DT. Typically, the condensed fuel layer thickness in a 100-.mu.-diameter sphere filled with 10 ng of DT is approximately 1 .mu.m.
Classical calculations to determine the surface shape of liquid DT held in a constant temperature spherical container in a gravitational field, and the time required for this equilibrium liquid distribution to be reached, were originally performed by Lawrence J. Campbell of the Los Alamos Scientific Laboratory in January of 1974. It was found that by minimizing the free energy, i.e., gravitational and surface energy, and using the appropriate wetting angle of the liquid, the liquid thickness monatomically increases from zero at the sphere top to a value much larger than average at the bottom, as graphically shown in FIG. 1. Campbell found the liquid sag time to be .tau.=3Dz/.rho.gt.sup.2, where D is the sphere diameter, g is the gravitational constant, and .rho., z, and t are respectively the liquid density, viscosity, and initial thickness. For a uniform 1-.mu.m-thick DT liquid film at 21 K covering the inner surface of a 100-.mu.m-diameter sphere, the sag time (.tau.) has been approximated at 4 s. The numerous difficulties encountered by experimentalists in producing desired cryogenic fuel cores, is evidenced by their numerous unsuccessful attempts at producing a successful target.
An early cryogenic target-producing technique pursued by T. M. Henderson, R. B. Jacobs, D. E. Solomon, G. H. Wuttke, at KMS Fusion, Inc. involved rapidly engulfing a fuel core in a droplet of liquid hydrogen. According to this technique, liquid or solid DT was produced in the fuel core, depending on the temperature of the hydrogen droplet refrigerant. This technique, however, produced grossly nonuniform deposits of DT which were clearly unsuitable for fusion target cores.
Another technique, conduction cooling through the fuel core support, has been experimentally studied as disclosed in "Laser Program at LASL, Jan. 1-June 30, 1975," Los Alamos Scientific Laboratory Progress Report LA-6050-PR (January 1976) pp. 80-81, and by T. M. Henderson, R. B. Jacobs, G. H. Wuttke, and D. E. Solomon in Advances in Cryogenic Engineering, Vol. 21 (1975). In accordance with that technique, the fuel core is glued to a cooled, thin metal fiber which provides refrigeration. With a metal fiber of suitable size, the DT's heat of vaporization and fusion, the heat radiation which has been absorbed and the heat generation due to the tritium .beta.-decay can be removed. DT fuel condenses in the glass microballoon near the support attachment and flows away from the attachment point until the condensed DT fuel reaches a location where the temperature is sufficiently high to cause evaporation. The evaporated DT recondenses at the coldest location and the cycle is repeated until a liquid layer is produced. With the proper fiber diameter, length, the correct amount of fastening glue, the right temperature and other conditions, a relatively uniform layer of liquid DT can be produced by this method of conduction cooling.
However, the layers so produced are transient. The experimental difficulty of first establishing and then maintaining the proper temperature profile across the fuel core hinders formation of a long-lived uniform liquid layer. A second disadvantage of the conduction cooling method, and of all methods that produce liquid, rather than solid fuel cryogenic fusion targets, is that a fraction of the fuel remains in a gaseous state. To minimize the amount of fuel in the gaseous state, the entire fuel core must be at a temperature proximate the fuel's triple point temperature. However, since all of the fuel rapidly freezes at locations where the fuel core is below the triple point, and since a large portion of the fuel core is above DT's evaporation temperature, the average temperature of the fuel core is many degrees higher than the fuel's triple point resulting in an appreciable fraction of the fuel remaining in a gaseous state. A third disadvantage of the conduction cooling method relates to the metal fiber support. To provide the necessary refrigeration for a 100-.mu.m-diameter glass microballoon, a wire diameter of approximately 15 .mu.m is needed. This support not only disturbs the desired spherical symmetry of the fuel core, but also adds considerable nonfuel mass which reduces the heating efficiency of the imploding laser beam, electron beam, or ion beam. A fourth disadvantage of the conduction cooling method results from the fractional distillation occurring within the fuel core. Since each molecular species of the DT fuel (T.sub.2,D.sub.2 or DT) has a different triple point and vapor pressure, and since the temperature difference from the support fiber to the opposite side is large, the fuel species will not be well mixed in the liquid layer.
Another technique pursued at Los Alamos by Stephen Sydoriak, utilizes heat removal through an exchange gas surrounding the fuel core in which the fuel core is attached with a thin glass fiber and slowly spun in quasi-isothermal conditions maintained by a helium heat exchange gas. Although complete liquid coverage of the interior fuel core surface is achieved, uniform layers of frozen DT are not formed as the temperature is decreased.
A passive method has also been pursued at Los Alamos as disclosed in "Laser Program at LASL, July 1-Dec. 31, 1975," Los Alamos Scientific Laboratory Report LA-6245-PR, p. 82, wherein solid DT layers in fuel cores have been tested. The concept of this technique comprises use of heat released by radioactive decay of the tritium fuel to assist in spreading an initially nonuniform solid layer. With the fuel core in a uniform temperature environment just below the triple-point of DT, the additional .beta.-decay heat in thick layer areas was expected to induce sublimation and subsequent recondensation in cooler, thin-layer areas. This procedure, however, was not found to be successful for reasons not well understood.
Another attempt to produce liquid-layer cryogenic fuel cores is disclosed by E. R. Grilly, "Condensation of Hydrogen Isotopes in Laser Fusion Targets," Rev. Sci. Instrum. 48, no. 2, 1977, pp. 39-42, wherein heat extraction is accomplished by a direct jet of gaseous helium at 4 K. In this cooling configuration, the effects of gravitationally driven liquid sag can be counteracted by imposing a temperature gradient on the fuel core. As with the conduction cooling technique, the jet method produces incomplete condensation of the fuel, as well as unstable and short-lived cryogenic fuel cores.
As is apparent, solid-fuel cryogenic fuel cores have a distinct advantage over the liquid fuel cores, in that solid fuel cores have a virtually unlimited lifetime and are very stable, provided the solid cores are kept well below the melting temperature of the core material. However, prior art devices and methods have been unable to produce uniform, solid, fuel core layers on target surfaces.